A conventional ring transmission line system can be represented by a local area network based upon the token ring technique as is disclosed, for example, in a journal "BIT", Vol. 16, No. 3, 1984.
According to this literature, a passive wiring concentrator or a station (ST) is directly connected to a ring subsidiary (called robe in this literature) of the line concentrator (called active ring concentrator (AWC) in this literature). In this literature, however, consideration has not been sufficiently given to the relay connection and to the structure thereof between the line concentrator and the ring subsidiary. That is, when the power source of the line concentrator connected to the main ring is turned off, a by-pass function of the line concentrator works to separate the ring subsidiary from the main ring in the line concentrator. Therefore, a terminal station connected to the ring subsidiary is not allowed to continue the communication.
According to the above-mentioned conventional art, a terminal station connected to the ring subsidiary is not allowed to continue the communication under the condition where the power source of the line concentrator is turned off, resulting in the interruption of the communication function of the terminal station connected to the ring subsidiary of the line concentrator.